The IOL seems to have more stringent standards (one needs to pass their in-house exam), which seems more reasonable than granting a membership to anyone willing to pay an annual fee.
This is not correct. You cannot become a Member of the ITI simply by "paying an annual fee". The requirements for ITI membership are generally regarded as being substantially stricter than those for IoL membership.
- what sort of value did their membership offer in terms of ?guarantee? for the client (i.e. was the membership giving a clear indication of someone's professional abilities)
Neither the ITI nor the IoL can give a "guarantee" for clients, but the ITI's membership requirements are geared much more specifically towards assessing candidates' ability to translate under real-world conditions. Also, if a customer has a grievance against a translator, he or she can take it up with the institute of which the translator is a member.
Now, the IOL person told me there should be a new ?Chartered Translator? diploma available
This is also not correct. "Chartered Translator" is not a diploma. It is a category of membership for IoL members who meet certain criteria.
The hint was, if you pass your exam and get your membership with the IOL now, you may automatically gain this Chartered Translator status, which would be, according to them, the only recognised professional qualification.
Again, this is not correct. As in most countries, the translation profession in the UK is open to anyone, whatever their qualifications. That is not going to change with the advent of IoL chartered status. There are many indications of competence and professional status, such as IoL or ITI membership, academic translation diplomas, etc., but the status of these "qualifications" varies among the buying public.
What would you recommend as a professional membership? Are they really worth the money?
Depends what you're looking for. If you just want some letters after your name to prove that you're a professional, probably not.